The discovery of cell-free fetal DNA in maternal plasma has opened up new possibilities for noninvasive prenatal diagnosis (Lo Y M D et al. Lancet 1997; 350:485-487). The mean/median fractional fetal DNA concentration has been reported to be approximately 3% to 10% (Lo Y M D et al. Am J Hum Genet 1998; 62:768-775; Lun F M F et al. Clin Chem 2008; 54:1664-1672). The fractional fetal DNA concentration is an important parameter which affects the performance of noninvasive prenatal diagnostic tests using maternal plasma DNA. For example, for the noninvasive prenatal diagnosis of fetal chromosomal aneuploidies (e.g. trisomy 21, trisomy 18 or trisomy 13), the higher the fractional fetal DNA concentration is, the higher will be the overrepresentation of DNA sequences derived from the aneuploid chromosome in maternal plasma. Indeed, it has been demonstrated that for every two times reduction in the fractional fetal DNA concentration in maternal plasma, the number of molecules that one would need to count to achieve aneuploidy detection would be four times (Lo Y M D et al. Proc Natl Acad Sci USA 2007; 104:13116-13121).
For the noninvasive prenatal detection of fetal trisomy by random massively parallel sequencing, the fractional fetal DNA concentration of a sample would affect the amount of sequencing that one would need to perform to achieve a robust detection (Fan H C and Quake S R. PLoS One 2010; 5:e10439). Indeed, a number of groups have included a quality control step in which the fractional fetal DNA concentration is first measured and only samples that contain more than a minimum fractional fetal DNA concentration would be eligible to generate a diagnostic result (Palomaki G E et al. Genet Med 2011; 13:913-920). Other groups have included the fractional fetal DNA concentration in their diagnostic algorithm for estimating the risk that a particular maternal plasma sample is obtained from an aneuploid pregnancy (Sparks A B et al. Am J Obstet Gynecol 2012; 206: 319.e1-9).
In addition to aneuploidy detection, the fractional fetal DNA concentration also similarly affects noninvasive prenatal diagnostic tests conducted using maternal plasma DNA for detecting monogenic diseases, e.g. the hemoglobinopathies (Lun F M F et al. Proc Natl Acad Sci USA2008; 105:19920-19925) and hemophilia (Tsui N B Y et al. Blood 2011; 117:3684-3691). The fractional fetal DNA concentration also affects the depth of sequencing that one would need to perform for constructing a fetal genomewide genetic and mutational map, as well as fetal whole genome sequencing (Lo Y M D et al. Sci Transl Med 2010; 2:61ra91 and U.S. Patent Application 2011/0105353).
A number of methods have been described for measuring the fractional fetal DNA concentration. One approach is to measure the concentration of a fetal-specific, paternally-inherited sequence that is absent from the maternal genome. Examples of such sequences include the sequences on the Y chromosome that are present in male fetuses and sequences from the RHD gene in a Rhesus D positive fetus carried by a Rhesus D negative pregnant woman. One could also measure the total maternal plasma DNA using sequences that are present in both the mother and the fetus. To arrive at a fractional fetal DNA concentration, one could then calculate the ratio of the concentration of the fetal-specific, paternally-inherited sequence over the concentration of the total maternal plasma DNA.
Another example of sequences that one could use includes the use of single nucleotide polymorphisms (Lo Y M D et al. Sci Transl Med 2010; 2:61ra91). A disadvantage of using genetic markers for the measurement of the fractional fetal DNA concentration is that no single set of genetic markers would be informative for all fetus-mother pair. Yet another method that one could employ is the use of DNA sequences that exhibit fetal or placental-specific DNA methylation patterns in maternal plasma (Nygren A O et al. Clin Chem 2010; 56:1627-1635). The potential disadvantage of the use of DNA methylation markers is that there may be inter-individual variation in the level of DNA methylation. Furthermore, methods that are used for the detection of DNA methylation markers are typically complex, including the use of methylation-sensitive restriction enzyme digestion (Chan K C A et al. Clin Chem 2008; 52:2211-2218) or bisulfite conversion (Chim S S C et al. Proc Natl Acad Sci USA 2005; 102:14753-14758) or methylated DNA immunoprecipitation (MeDIP) (Papageorgiou E A et al. Nat Med 2011; 17: 510-513).
Since the fractional fetal DNA concentration is an important value, it is desirable to have additional methods and systems for determining the value.